Programming Phoenix 1.4: Chapter 11 - Extracting Rumbl and RumbleWeb (Page 237, 238, 240)

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I couldn’t figure that out either. After following do book when I do
mix test

I get an error:

==> rumbl
** (Mix) Could not start application rumbl_web: RumblWeb.Application.start(:normal, []) returned an error: shutdown: failed to start child: RumblWeb.Presence
    ** (EXIT) shutdown: failed to start child: Phoenix.Presence.Tracker
        ** (EXIT) shutdown: failed to start child: RumblWeb.Presence_shard0
            ** (EXIT) an exception was raised:
                ** (ArgumentError) unknown registry: RumblWeb.PubSub
                    (elixir 1.11.4) lib/registry.ex:999: Registry.meta/2
                    (phoenix_pubsub 2.0.0) lib/phoenix/pubsub.ex:262: Phoenix.PubSub.node_name/1
                    (phoenix_pubsub 2.0.0) lib/phoenix/tracker/shard.ex:122: Phoenix.Tracker.Shard.init/1
                    (stdlib 3.14.2) gen_server.erl:417: :gen_server.init_it/2
                    (stdlib 3.14.2) gen_server.erl:385: :gen_server.init_it/6
                    (stdlib 3.14.2) proc_lib.erl:226: :proc_lib.init_p_do_apply/3